Abstract: Pathoqenesis and Therapy of Sideroblastic Anemia: We have recently reported on a novel murine anemia caused by deficiency of superoxide dismutase 2 (SOD2), that bears striking similarity to human sideroblastic anemia (SA). SA is a morphologically distinct group of disorders characterized by accumulation of excess iron within red cells during development. Genetic lesions responsible for several subtypes of SA have been recently elucidated, and in each case highlight the importance of mitochondria as a locus for heme biosynthesis, iron transport or iron homeostasis in developing red blood cells. SOD2, is a critical intra-mitochondrial catalytic ant/oxidant, and deficiency of this enzyme leads to late embryonic or neonatal lethality in mice, with pathologic evidence of widespread mitochondrial dysfunction including myopathy, neuropathy and metabolic derangement. In order to study cell-autonomous effects of SOD2 deficiency, we devised a transplantation system in which hematopeietic stem cells (HSC) from Sod2 null embryos were used to reconstitute the immune and hematopoietic tissues of lethally irradiated host animals, and found that a major phenotype resulting from loss of SOD2 is a hemolytic anemia. This result suggested that mitochondrial dysfunction secondary to increased oxidative stress, or perhaps direct oxidation of key target proteins during red cell development, may be central to the pathogenesis of SA. The importance of oxidative damage in this model of SA was further highlighted by the dramatic response to therapy with a novel class of ant/oxidants, catalytic SOD/catalase mimetics. A primary focus of this proposal is detailed characterization of pathology, biochemistry and protein/gene expression profiles in order to identify key molecular targets affected by loss of SOD2. A secondary focus is to document how catalytic ant/oxidant therapy affects this 'pathogenetic profile.' In parallel, we will examine gene expression profiles from marrow erythroid progenitors of SA patients, in part to classify this heterogeneous disorder, and in part to look for overlap with SOD2 deficiency. These studies will help to elucidate whether increased oxidative stress is a characteristic of SA, and thereby provide guidance as to the potential role of ant/oxidants as therapy for this disorder. The techniques developed in the course of this study--evaluation of protein oxidation and methods for purification of oxidized proteins--will provide tools for answering more general questions regarding the role of protein oxidation in other types of hemolytic processes, and as a determinant of survival of normal erythrocytes.